Long term survival of childhood cancers is now more than 70%. Anthracyclines, including doxorubicin, are some of the most efficacious anticancer drugs available. Their use has extended over 3 decades despite numerous side effects, of which cardiotoxicity is a significant problem. The studies of children 4 to 20 years after doxorubicin treatment have found significant cardiac insufficiency. The early effects of doxorubicin may be attributed to altered Ca+2 dynamics, increased oxidative stress, and altered myocardial energetics. However, the mechanisms underlying the delayed or late effects observed are unknown, but cardiac dysfunction leading to heart failure remains a significant long-term problem. Within the heart cardiomyocytes age, and are replaced by stem cells or cardiac progenitor cells. These stem cells/progenitor cells are continuously growing and replicating throughout the life of the organism. The major goal of this application is to determine if doxorubicin accelerates cardiac progenitor cell death and if this is antecedent to cardiac dysfunction. To accomplish this, mice will be injected with doxorubicin and the pathway to cardiac insufficiency identified. Mice will be examined post injection at times roughly equivalent to 2, 9, and 18 human years. Hypothesis: Doxorubicin-induced "late effects" on cardiotoxicity are the result of a failure of cardiac" pluripotent" cells. Cell specific failure is the result of mitochondrial damage and concomitant mitochondrial dysfunction which is cumulative and results from an inability to maintain the balance of repair/replacement that ultimately leads to activation of apoptotic pathways and cell loss. Objectives/Aims: 1) To identify which cell type(s) fail as the heart transitions from normal to overt heart failure following doxorubicin treatment. 2) To analyze of the impact of doxorubicin-induced mitochondrial dysfunction on cellular function. The technical goal of this RO3 application is to develop our competence in flow cytometry, which we believe will permit us to develop a more comprehensive application in the future. The biological goal of this application is to better understand the mechanisms underlying the "late effects" of doxorubicin-induced cardiotoxicity which are unknown. PUBLIC HEALTH RELEVANCE: Studies of children 4 to 20 years after doxorubicin treatment have found that many of these patients develop heart failure months and years after cancer treatment. We know that within the heart, the hearts cells age and need to be replaced by stem cells. The major goal of this application is to determine if doxorubicin accelerates cardiac stem cell death and if this is precedes heart failure.